Washington University in St. Louis has a rich tradition of radiation research and is a world leader in the field of radiation oncology for clinical care, a well as physics and biological research. Over the past twenty years, much emphasis has been placed on the development of instrumentation for the non-invasive imaging of biological processes in small animals, thereby providing investigators with a better understanding of the biology and the potential of therapeutic interventions in preclinical models. In this regard, Washington University is a recognized leader, with outstanding programs and instrumentation for preclinical magnetic resonance, nuclear, and optical imaging. Preclinical photon irradiators that can deliver radiation to precise anatomical locations in small animals are an important component of these research efforts. We therefore request funds to purchase the Xstrahl small animal radiation research platform (SARRP). Components of the SARRP include: a 225kV x- ray tube, x-ray controller, SARRP interface and PC, robotic specimen stage, safety interlock system, cone beam CT imaging system, and integrated shielding. Ten major users with NIH funding form the initial user base for the SARRP, along with three minor users. The PI has more than 25 years of expertise in clinical radiation oncology and in radiation and cancer biology research, with a primary focus is in drug development for treatment of cancer. He has assembled a strong team to oversee operations and QA of the instrument. In addition, Washington University has made a significant institutional commitment to ensure the successful use of the device after its installation. The irradiator will allow investigators to study basic questions regarding radiation research in an in vivo, pre-clinical model that will eventually lead to new paradigms in the way we treat disease in the clinical setting.